Current imaging spectrometer systems are generally either slit imaging spectrometers or bandpass-filter imaging spectrometers. However, slit imaging spectrometers must scan the scene spatially to build up a 2D image, and bandpass-filter imaging spectrometers must scan the scene spectrally. In contrast, computed tomography imaging spectrometers (“CTISs”) enable spectral imaging of transient events by capturing spatial and spectral information in a single snapshot. That is, CTISs capture spatial and spectral information from a two-dimensional (“2D”) scene in a single image frame. Spectra are obtained by means of tomographic reconstruction.
In a typical transmissive CTIS 30, as shown in FIG. 1, spots of light passing through a field stop 31 are collimated in a collimating lens 32, filtered through a wide-band filter 33, and passed through a 2D grating 34 which produces a 2D array of diffraction orders 35. A final focusing element, such as a re-imaging lens 36, re-images the various diffraction orders of light 37 onto a focal plane array (“FPA”) detector 38, e.g. a charge coupled device (“CCD”), that records the intensity of the incident light. Reflective-type CTISs operate in much the same manner except that a collimating mirror is used in place of the collimating lens.
Each diffraction order of light 37 transmitted from the grating 34 produces a spectrally dispersed image of the scene, except for the undiffracted “zeroth” order which produces an undispersed image in the center area of the FPA detector 38. The CTIS captures the scene's spatial and spectral information by imaging the scene through a 2D grating disperser. This produces multiple, spectrally dispersed images of the scene that are recorded by a focal plane array (“FPA”) detector. From the captured intensity pattern, computed-tomography algorithms can be used to reconstruct the scene into a cube of spatial (x and y) and spectral (wavelength) information.
The CTIS enables transient-event imaging spectrometry for applications including, for example: 1) spectral imaging of living biological systems that move/change rapidly during an experiment (e.g. cells, retina, colon, etc.); 2) industrial processes such as semiconductor etching; or 3) defense surveillance on regions in which neither the location nor the time of an explosion, missile launch, or chem-bio weapon deployment is known.
Current CTIS systems typically utilize a long focal-length collimation lens and a short focal-length re-imaging lens. The collimation lens is heavy, thereby significantly contributing to the total weight of the system. Further, the mechanical coupling of the collimation lens and the re-imaging lens adds weight away from the body. In addition, the use of both a collimation lens and a re-imaging lens presents issues associated with magnification, and the need to re-focus the re-imaging lens presents issues associated with calibration.